Genetic conditions

Genes are the instructions for the growth and development of our bodies. A genetic condition is caused by an altered gene or set of genes. There are four broad groups of genetic conditions: single gene conditions, chromosome anomalies, mitochondrial conditions and multifactorial conditions.

Genes are paired – one copy of each gene pair is inherited from the mother and the other from the father. Around 6,000 known genetic conditions are caused by inheriting an altered gene. Generally, the alteration means that the information contained in that gene is either changed or absent.

Single gene conditions#

A single gene condition results from an alteration in one gene. There are four main ways in which an altered gene can be inherited.

Autosomal dominant#

The alteration tends to be present in every generation because the altered copy of the gene is dominant over the healthy copy. An example is familial hypercholesterolaemia (genetically linked high cholesterol levels).

Autosomal recessive#

The affected person has two copies of the altered gene, having inherited an altered copy from both parents. They develop the condition because they do not have a functioning copy of the gene. Examples include cystic fibrosis, phenylketonuria (PKU) and sickle cell anemia.

X-linked recessive#

This type of condition is more common in males. It is caused by an alteration in a gene on the X chromosome. Because males usually have one X and one Y chromosome (XY), they do not have a second, healthy copy of the gene to compensate. Examples include Duchenne muscular dystrophy and hemophilia.

X-linked dominant#

This type of condition generally occurs in females. The “X” refers to one of the sex chromosomes, which determine sex. In most cases the mother provides an X, while the father provides either an X (female child, XX) or a Y (male child, XY). Women with an X-linked dominant condition have one altered copy and one normal copy of a gene on the X chromosome. An example is a rare form of rickets known as hypophosphataemic, or vitamin D resistant, rickets.

Chromosome anomalies#

Genes are the body’s instructions for making different molecules, such as proteins or hormones. The estimated 23,000 genes that make up a human being are arranged along tightly bundled strands of a chemical called deoxyribonucleic acid, or DNA. The DNA strands are packed into structures called chromosomes, which are present in every cell of the body.

Over 1,000 known conditions are caused by chromosome abnormalities. A chromosome condition means there is a change in either the structure or the number of chromosomes. This can happen in three main ways:

• the altered chromosome is passed from a parent to the child
• the anomaly happens when the sperm or egg (germ cells) is created
• the anomaly occurs soon after conception

Changes in the number of chromosomes#

Most people have 23 pairs of chromosomes, or 46 in all. When an egg or sperm is made, the pairs split so that each germ cell contains only 23 chromosomes. Occasionally an error occurs during this cell division. For example, the egg or sperm might be missing a chromosome (22) or have an extra one (24). This means that at conception the baby has either too few (45) or too many (47) chromosomes.

An example is Down syndrome, where a person has 47 chromosomes rather than 46. Babies are rarely born with changes in chromosome numbers, because most of these pregnancies end in miscarriage.

Changes in chromosome structure#

Sometimes the information in a chromosome breaks up and the pieces reform in a different pattern. For example, a fragment of a chromosome may break off and be lost during the formation of an egg or sperm cell. A section might break away and attach to another chromosome, a fragment may copy itself, or the ends of a chromosome may join to form a ring. Some structural changes are “balanced” (no chromosome material is lost or gained) and are unlikely to result in a genetic condition.

Uniparental disomy#

Uniparental disomy means a child has inherited a particular gene pair (both copies of the gene) from one parent only. This can cause a condition when it is important for the child to inherit one such gene from each parent.

Chromosomal mosaicism#

Normally every cell in the body contains the same genetic information – all 46 chromosomes, designated 46XX (female) or 46XY (male). A person with chromosomal mosaicism has different numbers of chromosomes in different cells; for example, 46 in some cells and 47 in others.

Mitochondrial conditions#

Mitochondria are like little batteries that make energy within each cell. The energy source is a chemical called adenosine triphosphate (ATP). Organs such as the brain, heart and liver cannot survive without ATP. Genes within the mitochondria, as well as in the nucleus of the cell, instruct the cell on how to make the enzymes that are crucial to ATP production. If any of these genes are altered, this can affect enzyme production and interfere with the production of ATP.

If one of the genes in the mitochondria is altered, the condition is inherited only from the mother. This is because each person inherits their mitochondria only from their mother, not from their father.

Depending on the genes involved, the symptoms of a mitochondrial condition can affect different parts of the body:

• the brain and spinal cord, which can cause intellectual disabilities, deafness, vision problems and seizures
• the heart, which can cause cardiomyopathy (heart failure) and irregular heartbeat
• the muscles, which can cause poor muscle tone and floppiness

Multifactorial conditions#

Multifactorial conditions involve several factors. Many common birth conditions, and diseases such as high blood pressure, are caused by the environment interacting with the action of many genes. This is sometimes called polygenic inheritance.

For example, the birth condition spina bifida is caused by the action of several genes and also depends on the amount of folate in the mother’s diet during pregnancy. High blood pressure is influenced by many genes, but also by a person’s diet and salt intake.

Genetic counseling#

If you, your child or another family member has been diagnosed with a chromosomal or genetic condition, or if you know that a condition runs in your family, it may help to speak to a genetic counselor. Genetic counselors are health professionals qualified in both counseling and genetics.

As well as providing emotional support, they can help you understand a condition and its cause, how it is inherited (if it is), and what a diagnosis means for your or your child’s health and development, and for your family. They are trained to provide information and support that is sensitive to your family circumstances, culture and beliefs.

If a genetic condition runs in your family, a genetic counselor can explain what genetic testing options are available to you and other family members. You may also choose to see a genetic counselor if you are planning a family, to learn your risk of passing a condition on to your child or to arrange prenatal tests. A counselor can also connect you with other individuals and families affected by the same condition.

Key points#

• A genetic condition is caused by an altered gene or set of genes.
• Around 6,000 known genetic conditions are caused by inheriting an altered gene.
• In autosomal dominant conditions, the altered copy of the gene is dominant over the healthy copy.
• In autosomal recessive conditions, people develop the condition because they do not have a functioning copy of the gene.
• X-linked recessive conditions are more common in males.

Where to get help#